Feather Selection with Non-Uniform Radii in GIMP

GIMP’s default Feather command only applies a symmetrical blur radius across an entire selection. To achieve non-uniform horizontal and vertical feathering, you must convert your selection into a Quick Mask, apply an unlinked Gaussian Blur to control the X and Y axes independently, and convert the mask back into an active selection. This guide walks you through each step required to accomplish this effect.

Step 1: Make Your Initial Selection

Create the selection you want to feather using any of GIMP's standard selection tools, such as the Rectangle Select Tool, Ellipse Select Tool, or Free Select Tool.

Step 2: Enable the Quick Mask

Convert your selection into an editable raster mask:

  • Press Shift + Q, or
  • Navigate to the top menu and select Select > Toggle Quick Mask, or
  • Click the small dashed square icon in the bottom-left corner of the image window.

Your unselected areas will be tinted red by default, while your selected area remains clear.

Step 3: Open the Gaussian Blur Filter

With the Quick Mask active, you can modify the selection boundary using standard raster filters:

  • Go to Filters > Blur > Gaussian Blur... in the main menu.

In the Gaussian Blur dialog window:

  • Locate the Size X and Size Y input fields.
  • Click the chain link icon located directly to the right of the Size values to break the link. Breaking this link allows the horizontal (X) and vertical (Y) blur amounts to change independently.

Step 5: Set Your Custom Radii

  • Enter your desired horizontal feather amount in the Size X field.
  • Enter your desired vertical feather amount in the Size Y field.
  • Click OK to apply the directional blur to the Quick Mask.

Step 6: Return to the Selection

Convert the blurred mask back into an active selection:

  • Press Shift + Q, or
  • Go to Select > Toggle Quick Mask.

The Quick Mask overlay will disappear, leaving an active selection feathered with your chosen non-uniform horizontal and vertical dimensions.